The present invention relates to a feed pump with a variable-speed drive for metered dispensing of a feed quantity, the feed pump being constructed as a single-stage centrifugal pump with a radial wheel having a centrifugal type of construction, arranged for rotation, without a sealing gap, in an impeller chamber of a pump casing, in order to convey a fluid between a pump inlet and a pump outlet.
In the research and development process sector of the chemical and pharmaceutical industry, there is a continuing need for ever faster developments at lower cost. The production of such materials demands more flexible, smaller-scale and more environmentally friendly processes. This leads to the use of process engineering components which are operated at partial capacity with very low filling volumes and with a continuous material flow. Due to the requirement for flexible use of such plants, the overall plant, together with the assemblies mounted in it, must have a high scavenging capability with the aid of special scavenging media.
Such plants require an accurate, constant, freely adjustable and pulsation-free volume flow of liquid materials. For highly accurate continuous volume flows in the range of zero milliliters per minute up to a three-digit number of liters per hour, positive-displacement pumps in the form of micro toothed-ring and gear pumps and in the form of diaphragm and piston pumps are used. Such positive-displacement pumps have the disadvantage of the poor reliability as a result of friction between the components to be sealed off, moved in relation to one another, and their pulsating feed stream. The expense in terms of maintenance caused thereby and the costs of wearing parts and of changing these are an impediment to rapid research and development work and appreciably disrupt the production process.
U.S. Pat. No. 6,986,647 (=WO 2005/052365) discloses a centrifugal pump constructed as a canned motor pump, for circulating supercritical hydrocarbons. The drive motor has a can composed of polyetheretherketone (PEEK), within which is arranged a rotor protected by a high-grade steel covering. Ceramic bearings of the pump shaft and of the drive motor are lubricated by a partial-stream, withdrawn from the pump casing, of the feed fluid. The impeller, of open design, has a diameter between 1 and 2 inches, and the rotor, driving the impeller, of the rolling-bearing-mounted direct-current motor has a diameter between 1.5 and 2 inches. The single-stage pumping device with the open impeller is to reach maximum rotational speeds of up to 60,000 rev/min. The suction connection piece, the delivery connection piece and a type of spiral space following the impeller are arranged in an outer pump-casing part, while an inner pump-casing part has the overhung-mounted impeller and a fastening for a variable-speed direct-current canned motor as a drive motor.
This canned-motor design has the disadvantage of the multiplicity of slots which, because of the complex flow routing between the pump and canned motor, greatly impede cleaning of the pump. Since part of the feed fluid flows permanently through the motor and its can space, the frictional heat of the rolling bearings and the heat loss from the canned motor give rise to an undesirably high introduction of heat into the feed fluid.